Zeolitic materials, both natural and synthetic, have been demonstrated to have catalytic properties for various types of hydrocarbon conversions. Certain zeolitic materials are ordered, porous crystalline aluminosilicates having a definite crystalline structure within which there is a large number of uniformly sized channels, whose pore dimensions will accept for adsorption molecules of certain dimensions while rejecting those of larger dimensions. They have been defined as a rigid three-dimensional framework of SiO.sub.4 and AlO.sub.4 in which the tetrahedra are cross-linked by the sharing of oxygen atoms so that the ratio of the total aluminum and silicon atoms to oxygen is 1:2. The electrovalence of the tetrahedra containing aluminum is balanced by the inclusion in the crystal of a cation, for example, an alkali metal or an alkaline earth metal cation. They are susceptible to cation exchange, by which it is possible to vary the properties of a given aluminosilicate by suitable selection of the cation. The spaces between the tetrahedra are usually occupied by molecules of water prior to dehydration. Recently, zeolite materials having very high ratios of silicon to aluminum atoms have been prepared, as described, for example, in European patent application No. 14059. Certain zeolites of synthetic origin may be prepared by hydrothermal crystallization from a composition containing water, the elements of sodium oxide, an oxide of silicon and a source of nitrogen-containing organic cations. A source of aluminum oxide may optionally be present in an amount dependent upon the silica:alumina ratio desired in the final product. Zeolites which have been prepared in this way include zeolites ZSM-5, ZSM-11, ZSM-12, ZSM-23, ZSM-35, ZSM-38 and ZSM-48 which are described, respectively, in U.S. Pat. Nos. 3,702,886 (ZSM-5); 3,709,979 (ZSM-11); 3,832,449 (ZSM-12); 4,076,842 (ZSM-23); 4,016,245 (ZSM-35); 4,046,859 (ZSM-38) and European Pat. No. 15132 (ZSM-48). Reference is made to these patents for details of these zeolites, their preparation, properties and uses.
The identity of the zeolite which is produced from the solution depends in part upon the organic cation which is present in the mixture. Thus, ZSM-5 is normally produced in the presence of tetrapropyl ammonium (TPA) ions, ZSM-11 in the presence of tetrabutyl ammonium (TBA) ions, ZSM-12 in the presence of tetraethyl ammonium ions and ZSM-23, ZSM-35 and ZSM-38 in the presence of ethylenediamine, pyrrolidine or 2-hydroxyalkyl trialkyl ammonium ions, as described in the patents referred to above. Other organic cations have yielded other zeolites, for example, the synthetic zeolite described in U.S. Pat. No. 3,699,139 is prepared in the presence of benzyltrimethyl ammonium cations.
In their as-synthesized form, the zeolites frequently contain large amounts of alkali or alkaline earth metal ions which are derived from the synthesis composition. Because the presence of these ions is often undesirable if the zeolite is to have satisfactory catalytic activity, they must be removed by ion-exchange before the zeolite is ready for use. If the content of the alkali or alkaline earth metal ions is to be reduced to an adequately low figure, usually less than 0.05 weight percent, repeated and lengthy ion-exchange may be necessary. These steps are expensive to carry out and it would therefore be desirable to find a way of preparing a zeolite directly in a form which has a low content of alkali metal cations.
These difficulties are compounded if the zeolite contains bulky organic cations and in such cases, even lengthy ion exchange procedures may not be sufficient to remove the alkali metal cations. Removal of the organic ions, e.g. by calcination, prior to ion exchange is therefore often required.